Several hundred thousand people die in the United States each year from acute myocardial infarction, and many more suffer from chronic heart problems. A major contributing factor in both acute and chronic heart problems is a reduction in nutrient blood flow to the muscles of the heart resulting from a reduction of blood flow through the coronary blood vessels. The reduction in flow may be caused by deposits of atherosclerotic plaque on the walls of the blood vessel, which causes a narrowing of the lumen or channel of the blood vessel. When the lumen is sufficiently narrowed, the rate of flow of blood may be so diminished that spontaneous formation of a thrombus or clot occurs by a variety of physiologic mechanisms. As is known, once a blood clot has started to develop, it extends within minutes into the surrounding blood, in part because the proteolytic action of thrombin acts on prothrombin normally present, tending to split this into additional thrombin which causes additional clotting. Thus, the presence of atherosclerotic plaque not only reduces the blood flow to the heart muscle which it nourishes, but is a major predisposing factor in coronary thrombosis.
Among the treatments available for the conditions resulting from plaque formations are pharmacological means such as the use of drugs, for example nitroglycerin, for dilating the coronary blood vessels to improve flow. In those cases too far advanced to be manageable by drugs, surgical treatment may be indicated. One of the surgical techniques commonly used is the coronary bypass, in which a substitute blood vessel shunts or bypasses blood around the blockage. The bypass operation is effective, but is expensive and subject to substantial risks.
Another treatment for plaque formations is mechanical removal by means of a rotary cutter catheter, as described, for example, in U.S. Pat. Nos. 4,445,509 and 4,990,134, issued May 1, 1984 and Feb. 5, 1991, respectively, both in the name of Auth. Catheters are known in which a cutter can be driven at speeds as great as 200,000 rpm. When the cutter is applied to the arterial walls, the walls may undesirably be perforated.
Percutaneous transluminal balloon coronary angioplasty is a widely used alternative to open-heart coronary bypass surgery for the treatment of acute and chronic heart problems. This method involves insertion of a deflated balloon into the lumen of an artery partially obstructed by plaque, and inflation of the balloon in order to enlarge the lumen. The lumen remains expanded after removal of the catheter, but the obstructing material remains. Among the problems with this technique, as described in the article "Ultrasonic Plaque Ablation," by Siegel et al., published at pp 1443-1447 of Vol. 78, No. 6, December 1988 issue of the periodical Circulation, are those involved in introducing the catheter with its balloon into a blood vessel which is completely or almost completely occluded, and restenosis of the narrowed vessel after the angioplasty procedure by recurrence of the arterial plaque.
Microwave aided balloon angioplasty is described in U.S. Pat. No. 4,643,186 issued Feb. 17, 1987 in the name of Rosen et al. In the arrangement as described by Rosen et al., a catheter including a microwave transmission line terminates at its distal end in an antenna surrounded by a balloon. During angioplasty, microwave power is applied to the proximal end of the catheter and flows to the antenna, which radiates the energy to the plaque for heating and thereby softening the plaque. The balloon is expanded against the softened plaque to thereby expand the lumen of the blood vessel. While microwave heating improves balloon angioplasty, the plaque is not removed by the angioplasty, and may expand after the procedure, or if it does not expand, may provide a base upon which additional plaque may be deposited.
Another technique which has recently received a good deal of attention is transluminal laser catheter angioplasty. This treatment involves introduction into the coronary artery of a fiber optic cable, the proximal end of which is connected to a laser energy source. The distal end of the fiber optic cable is directed towards the plaque. The laser is pulsed, and the resulting high energy light pulse vaporizes a portion of the plaque. Many problems remain unsolved in laser catheter angioplasty, as in mechanical cutting catheters. When the energy of the laser discharge is directed towards the arterial walls, the walls may undesirably be perforated. Further problems relate to the difficulty in matching the characteristic of lasers and fiber optic cables to the frequency absorption characteristics of various types of plaque, and the by-products of the destruction of the plaque.
Experimental studies have shown that ultrasound or acoustic angioplasty has the potential for differentiating between normal arterial walls and abnormal walls including atherosclerotic plaques and thrombi, as described, for example, in "Experimental Ultrasonic Angioplasty:Disruption of Atherosclerotic Plaques and Thrombi in Vitro and Arterial Recanalization in Vivo," by Rosenschein et al., published at pp 711-717 of Vol. 15, Mar. 1, 1990 issue of the J. Am. Coll. Cardiology, and in "Ability of High-Intensity Ultrasound to Ablate Human Atherosclerotic Plaques and Minimize Debris Size," by Ernst et al., published at pp 242-246 of Vol. 68 of The American Journal of Cardiology, Jul. 15, 1991. It appears that significant ultrasonic energy must be applied to the plaque in order to effect its removal. U.S. Pat. No. 3,565,062, issued Feb. 23, 1971 in the name of Kuris, describes an ultrasonic catheter including an electrodynamic, piezoelectric or magnetostrictive ultrasonic motor operating in the range of 1000 Hz to 100 KHz, which may also be operated in a swept-frequency mode. The vibrations from the motor are coupled, through an elongated transmission member which extends through the catheter, to a vibrating tool or head, shaped for removal of plaque. As described therein, when the device is operated at a fixed frequency, nodes along the transmission member are heated. U.S. Pat. No. 5,163,421, issued Nov. 17, 1992 in the name of Bernstein et al. describes the problem of heating of the transmission member, and reduction of the power transmitted to the tool due to transmission losses in the transmission member. The solution suggested in the Bernstein et al. patent is the use of a high Q material. However, even with the use of high-Q transmission members, losses in the vicinity of 50% (-3 dB) occur in ultrasonic catheters of the lengths necessary for coronary angioplasty, and these losses increases significantly at bends in the transmission member.
Improved angioplasty catheters are desired.